Methods of and apparatus for producing a visual record of physical conditions of materials traversed by a borehole



Feb. 20, 1968 J. ZEMANEK, JR 3,369,626

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Feb. 20, 1968 J. ZEMANEK. JR 3,369,626

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$\GNAL CONTROL VOLTAGE GATE United States Patent O ABSTRACT OF THE DISCLOSURE The specification discloses an acoustic logging system for producing visual records of the physical conditions of wall structure traversed by a borehole. A beam of pulsed acoustic, high-frequency energy is swept across the face of the wall structure through 360. Reflected energy is detected and sent uphole to the cathode ray oscilloscope. The sweep of the cathode ray beam is initiated by a geographical sensing device in the downhole tool so as to correlate the rotational position of the beam with the received data. The'cathode ray beam is moved as a function of depth relative to a recording medium to produce a picture of the wall structure.

This application is a continuation-in-part of application Serial No. 391,151, filed Aug. 21, 1964, now abandoned.

This invention relates to methods of and apparatus for examining boreholes and formations adjacent boreholes and, more particularly, relates to an improved system including a source of pulsed energy, a receiver for detecting the reflections of said pulsed energy, and a display device for converting the received pulses into a representation of the configuration of the borehole or of lithology of adjacent formations and of any anomalies therein.

It is frequently necessary to measure the diameter, or cross-sectional area, of a borehole which may extend to great depth. In addition to determining the actual configuration of the borehole, it is desirable to determine anomalies which may exist at different depths in the borehole. For example, these anomalies may be a fault or crack in the borehole, a fracture in formations traversed by the borehole, or the presence of an obstruction in the borehole. In other situations, it is desirable to determine the existence of faults, or other characteristics, in the adjacent formations spaced from the borehole.

One type of borehole logging system useful for making such determinations includes a transmitter of pulsed energy which, for example, may be a transmitter of supersonic pulses. This transmitter and a receiver adapted to detect the energy pulses after reflection from the walls of the borehole are included in an assembly which is moved through the borehole. Concurrent with the movement through the borehole, the assembly is rotated about the borehole axis so that the reflected energy signals are indicative of the configuration and anomalies of theborehole at different angular positions. The detected signals are converted to a presentation which displays either the configuration or the anomalies in formations traversed by the borehole, or both. Or, the detected signals are recorded for subsequent use.

In accordance with one aspect of this invention, there is provided a technique for most usefully presenting the signals representing the configuration and anomalies of the formations surrounding the borehole as determined by the reflected energy pulses. (As used herein, the formations surrounding the borehole includes the borehole casing as well as the adjacent geophysical formations.) As examples, there is produced a display having three dimension effect which represents the configuration and anomalies of the borehole over a segment of some depth. Other displays representing only the anomalies in the borehole and some representing both the anomalies and the configuration, or diameter, of the borehole are provided in accordance with this invention.

In addition to conducting the reflected energy signals to the indicating device, which is generally located uphole, there must additionally be provided means for conducting to the uphole location signals indicative of the angular position of the tool. By applying both the signals indicative of the angular position and the reflected energy signals to the indicating device, a display can be produced which represents quite accurately the configuration of the borehole at a particular depth.

In the prior art, a synchro system has commonly been provided for indicating the angular position of the transmitter-receiver assembly. As the assembly is rotated by a motor, a synchro-transmitter is also rotated. At the uphole location, a synchro-receiver is provided and both the synchro-transmitter and the synchro-receiver are energized from a source of AC power. Three signal leads connect the downhole synchro-transmitter with the uphole synchro-receiver and these three connections carry signals which act to drive the synchro-receiver to the same angular position as that assumed by the synchro-transmitter. Such a system has disadvantages under certain conditions. First, three signal leads are required to connect the synchro-transmitter with the synchro-receiver. Where the tool is to be lowered to great depth in the borehole, the provision of all of these conductors extending from the uphole location to the downhole tool presents difficulties. Furthermore, the requirement that the downhole synchro-transmitter be supplied with AC power introduces problems of cross-feed of the AC to other signal-carrying conductors which extend to the downhole tool.

In accordance with another aspect of this invention, the above-mentioned problems of synchro systems are obviated by utilizing a potentiometer as a downhole angular position indicating device. The movable contact of the potentiometer is driven concurrently with the rotation of the transmitter-receiver assembly. The voltage on this movable contact is applied to a single conductor which extends to the uphole location and the voltage on this conductor is indicative of the angular position of the transmitter-receiver assembly. At the uphole location, this voltage is applied to a self-balancing servo system which drives the movable contact of an uphole potentiometer to a position which corresponds with the angular position of the downhole transmitter-receiver assembly. This uphole potentiometer can be used to produce horizontal and vertical deflection voltages which are applied to an indicating device of the electron beam type so that the electron beam traces out paths on the face of the indicating device corresponding with dilferent angular positions of the borehole.

In accordance with other aspects of this invention, there are provided improved potentiometer circuits and self-balancing circuits which are not susceptible to dead spots or ambiguous changes in the direction of rotation of the indicating potentiometer.

In accordance with another aspect of this invention, the foregoing is accomplished by providing an uphole potentiometer having two movable contacts and which is wound so that the voltages developed at the two contacts vary in a sinusoidal manner. Further, the contacts are positioned apart so that the sinusoidal voltage developed on one contact lags the voltage on the other contact by 90. The voltages on these two movable contacts are converted to horizontal and vertical deflection voltages for the indicating device by means of two sampling circuits which periodically apply the sinusoidal voltages to two capacitors. When the voltage is applied to these capacitors, the voltage across the capacitors rises linearly toward the sampled voltage. Such a system has the advantage that it is not necessary to apply a changing sweep voltage directly to the potentiometer as was the case with many prior art systems of this type. This prior art arrangement is quite unsatisfactory, particularly where a short sweep is desired, for the reason that all potentiometers have a certain capacitance and this capacitance distorts the waveform of the applied sweep voltages.

In accordance with another aspect of this invention, there is provided a technique for utilizing an acoustic logging tool to determine the nature and character of subsurface formations and the presence of anomalies therein and there is provided a technique wherein the acoustic logging tool may be utilized to determine the dip, or slope, of subsurface formations which are traversed by a borehole.

In accordance with another aspect of this invention, there is provided a borehole logging system for probing formations defining a borehole with a beam of high frequency energy and including means for receiving energy reflected from said formations, and means for displaying the received energy on a recording medium in correlation with change in depth of said logging system in said borehole so that there is registered on said recording medium a representation of the characteristics of said formations at different depths.

In accordance With another aspect of this invention, there is provided an uphole sweep generator which generates a sweep voltage free of undesirable ripple to produce a more accurate display on the uphole display medium.

In accordance with another aspect of this invention, the sweep voltage for the display medium is initiated by a signal from a magnetic field sensitive device in the downhole tool so that the beginning of the sweep corresponds with the orientation of the rotating beam of high frequency energy in a particular geographic direction.

In accordance with a still further aspect of this invention, there is provided a circuit for detecting the high frequency receiver signal to convert it to a signal of lower frequency for intensity modulation of the display device.

In accordance with a still further aspect of this invention, there is provided an automatic gain control circuit for providing a constant intensity of modulation of the electron beam by the receiver signal when the logging tool is being used to detect faults or cracks in the borehole wall.

The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended claims in conjunction with the drawings in which:

FIG. 1 shows a sonic borehole calipering system;

FIG. 2 shows a borehole calipering system in accordance with the present invention:

FIGS. 2a and 2b show the two outputs of the sine-cosine potentiometer;

FIGS. 20-27 are waveforms depicting the operation of the system of FIG. 2;

FIG. 2g is a representation of the face of the display device;

FIG. 3 shows the circuitry for producing the deflection voltages;

FIG. 4 shows a modification of the potentiometer and self-balancing system;

FIG. 4a is a waveform of the voltage produced on the movable contact of the downhole potentiometer;

FIG. 5 shows another modification of the potentiometer and self-balancing system;

FIG. 6 shows a display produced in accordance with one modification of the invention;

FIGS. 6a6e are waveforms depicting the operation of the modification used to obtain the display of FIG. 6;

FIG. 7 shows the circuitry used in obtaining the display of FIG. 6;

FIG. 8 shows a display obtained in accordance with another modification of the invention;

FIG. 8a shows a waveform of the horizontal deflection voltage of a modification of the invention;

FIG. 9 shows another modification of the circuitry used to obtain a display;

FIGS. 9a9e are waveforms depicting the operation of the modification of the circuitry used to obtain the display of FIG. 9;

FIG. 9] is the display obtained in accordance with the modification of FIG. 9;

FIG. 10 shows downhole circuitry for producing an indication of angular position of the transmitter-receiver assembly;

FIGS. Ila-11f are waveforms depicting the operation of the angular position indicating circuitry;

FIG. 12 shows the uphole circuitry used to indicate angular position and depth of the logging tool;

FIG. 13 shows in more detail the Hall effect device for indicating angular position;

FIG. 14 shows the circuit details of the tunnel diode detector;

FIG. 15 shows the circuit details of the gates of FIG. 12;

FIG. 16 shows a borehole traversing a dipping formation;

FIG. 17 shows a display indicating the dip of formations traversed by a borehole;

FIG. 17a shows a display indicating the dip of a fault traversed by a borehole;

FIG. 18 shows another embodiment of the downhole tool forming this invention;

FIG. 19 shows the uphole circuitry to be used with the downhole tool of FIG. 18;

FIG. 20 shows a display produced by the embodiment of FIGS. 18 and 19;

FIG. 21 is a block diagram of automatic gain control circuitry; and

FIG. 22 is a circuit diagram of the automatic gain control.

Referring now to FIG. 1, there is shown a common arrangement for rotating a sonic transmitter and receiver in a borehole. As is normal in acoustic logging systems, a transmitter 1 and a receiver 2 are enclosed in a logging tool 3 which is lowered into the borehole. The transmitter 1 and receiver 2 are rotatably mounted so that they can be continuously rotated by the motor 4. In order to transmit a signal uphole, which is indicative of the angular position of the transmitter 1 and receiver 2, a synchro transmitter 5 is provided. The motor 4 and the synchrotransmitter 5 are supplied with AC power from the power supply 5a located uphole.

As is well known in the synchro system art, the synchro transmitter 5 provides signals on the leads 6, 7 and 8 which are indicative of the angular position of the synchro transmitter 5. These signals are applied to a synchro receiver 9 located uphole. The same AC voltages from power supply 5a are also applied to the synchro receiver 9. The signal leads 6, 7 and 8 drive the synchro receiver 9 to the same angular position to which the synchro transmitter 5 has been driven. Therefore, the synchro receiver 9 is in an angular position which matches the angular position of the synchro transmitter 5. In order to convert this position to a voltage indicative of the angular position, a potentiometer 10 is provided.

The system just described has the disadvantage that three leads 6, 7 and 8 for the synchro system, plus two leads for the AC power supply must be provided between the uphole location and the downhole logging tool. The system shown in FIG. 1 also has the disadvantage that the AC power voltage results in cross-feed to the transmitter and receiver signals on the other cables going uphole.

Both of the foregoing disadvantages are obviated in accordance with one aspect of my invention by providing a system such as is shown in FIG. 2. In FIG. 2, instead of providing a synchro system for indicating the angular position of the downhole transmitter and receiver, there has been provided an arrangement by which a varying DC voltage on a single line indicates the angular position. Also, only two DC power voltage leads are connected to the downhole logging tool. By doing this, the total number of leads required to supply energizing power and to indicate the position of the downhole tool has been reduced from five, in the case of the synchro system, to three in the system shown in FIG. 2. In addition, since no AC power is required in the downhole tool, there is less cross-feed and the resultant operation is very much improved.

A transmitter 11 and a receiver 12 are rotatably mounted in the logging tool 13. While a separate transmitter and receiver have been shown, it will be appreciated that a single transducer, commonly referred to as a transceiver, could be used to perform both functions. The transmitter 11 and receiver 12 are continuously rotated by the motor 14. The transmitter 11 is periodically pulsed by the sync pulse generator 11a. It will be understood, of course, that the transmitter may be energized in any repetitive manner which need not necessarily be periodic in time. When the transmitter 11 produces a pulse of acoustic energy, this pulse travels to the periphery of the borehole and is returned as an echo to the reeciver 12. The receiver 12 produces a pulse which occurs at a time after the transmitter pulse which is indicative of the distance from the center of the borehole to the periphery 'of the borehole from which the echo was received. The receiver pulse from receiver 12 is applied to an amplifier 12a. In FIG. 2, the sync pulse generator 11a and the amplifier 12a have been shown at an uphole location for convenience of drawing. However, normally, sync pulse generator 11a and amplifier 12a will be contained in the downhole logging tool 13. The output of amplifier 12a is used to turn on the beam of a plan position indicator 26. The output of amplifier 12a is applied to the CRT cathode input of the oscilloscope to modulate the intensity of the beam in accordance with the received signal. As the transmitter 11 and receiver 12 rotate in the borehole, the transmitter 11 is continuously pulsed thereby providing at receiver 12 a return from each section of the periphery of the borehole. When the transmitter 11 and receiver 12 have been rotated a full 360, there have been produced signals at the receiver 12 which are indicative of the size of the borehole and also indicative of any faults or breaks in the wall of the borehole.

' As the assembly, including transmitter 11 and receiver 12 rotates, the movable contact of the potentiometer 15 also rotates. A DC voltage from the DC voltage supply 16 is connected across the potentiometer 15. As the movable contact of potentiometer 15 rotates, a slowly varying DC voltage is produced on the lead 16a. This varying DC voltage is applied to the movable contact 17 of the balancing potentiometer 18 located uphole. Potentiometer 18 is connected in a balancing system such that the position of movable contact 17 will always exactly match the position of the movable contact of the potentiometer 15 located downhole. This balancing system works as follows: As the voltage on the movable contact 17 changes, the voltage at the point 19 changes. This changing voltage at the point 19 is connected to an amplifier 20 which drives the balancing motor 21. The balancing motor 21 in turn drives the movable contact 17 in a direction which reduces the voltage at the point 19 to zero. In this manner, the movable contact 17 is driven to a position which matches the position of the movable contact of the potentiometer 15.

The motor 21 concurrently drives the movable arms 22 and 23 of the sine-cosine potentiometer 24. This potentiometer is of the type which produces a voltage which varies sinusoidally as the movable contact rotates. Such potentiometers are readily available. For example, Helipot 6 model No. 5713 R20k C.5 (sine-cosine) can be used for this purpose. The voltage produced at the movable contact 22 is a sinusoidal voltage such as that shown in FIG. 2a. The voltage produced at the movable contact 23 is a cosine voltage such as that shown in FIG. 2b. The movable contacts 22 and 23 are apart so that the voltage produced on movable contact 22 lags the voltage produced on movable contact 23 by 90. The voltages on the movable contacts 22 and 23 are indicative of the angular position of the downhole transmitter 11 and receiver 12.

In order to convert the sine and cosine voltages to deflection voltages which may be used to drive the deflection plates of a plan position indicator 26 (PPI), the sinusoidal voltage on the movable contact 22 is applied to a second sampling amplifier 25a and the cosine voltage on the movable contact is applied to a first sampling amplifier 25.

Before proceeding with the description of the circuitry of FIG. 2 which converts the sine and cosine voltages to horizontal and vertical deflection voltages for the plan position indicator 26, it is desirable to examine the nature of the voltages which must be applied to the horizontal and vertical deflection plates of the plan position indicator 26.

Referring to FIG. 2g, there is shown the face of the plan position indicator in more detail. Each time the transmitter 11 is fired, an electron beam sweeps outwardly from the center of the indicator toward the periphery. Such a sweep may follow the path 27. When the echo of the transmitter pulse is received at receiver 12, receiver 12 produces a signal which turns on the beam at the point 28 to indicate the periphery of the borehole at the point 28. As the transmitter 11 is rotated in the borehole, it is successively fired.

As the transmitter is fired at different angular positions, the plan position indicator 26 traces out successive sweeps starting from the center and extending toward the periphery of the indicator. Each successive sweep must be at a different angular position. During these successive sweeps at different angular positions, the electron beam will be turned on at positions indicative of the distance from the center of the boreholeto the periphery of the casing at the successive angular positions. In this manner, there will be traced out on the plan position indicator a visible trace 29 indicative of the shape of the borehole at that depth. Further, if there is an anomaly such as a fault or crack in the borehole at that depth, the electron beam will not be turned on during that sweep. This is indicated by the discontinuities 30 in the trace. Therefore, it would be quite easy to determine the location of the fault in the borehole at that depth from the visible trace on the plan position indicator.

Returning to the voltages necessary to generate successive sweeps of the electron beam on the plan position indicator 26, refer again to the sweep 27 made by the electron beam. In order to sweep the electron beam along the path 27, it is necessary to apply a voltage to the horizontal deflection plate which starts at zero and increases linearly toward a voltage proportional to the distance E on FIG. 2g. Similarly, a voltage must be applied to the vertical deflection plates which starts at Zero and increases linearly to a voltage proportional to E The voltages proportional to E and E which voltages will subsequently be referred to as E and E are provided by the sine-cosine voltages from the movable contacts 22 and 23. For example, referring to FIGS. 2a and 2b, assume that the transmitter has been fired at a time 31 which may be expressed by the angular coordinate 0. At that time the voltage on movable contact 22 is equal to the voltage E and the voltage on movable contact 23 is equal to E These voltages are sampled at the time 31 and there is produced the horizontal and vertical deflection voltages which increase linearly toward E and E as just described. In this manner, the electron beam is 7 caused to move along the path 27 on the face of the indicator 26.

When the transmitter 11 is fired by the sync pulse generator 11a, there is coincidentally produced a sync pulse which is applied to delay generator 32. The function of delay generator 32 in producing a different display will subsequently be explained. The sync pulse passes through delay generator 32 to trigger the monostable multivibrator 33. The monostable multivibrator 33 produces a positive-going step voltage as shown in FIG. 2a.. The step voltage goes positive at the time 34 which is coincident with the occurrence of the transmitter pulse T shown in FIG. 2c. The output of monostable multivibrator 33 remains up until the time 35, at which it returns to its more negative position. The interval of time between 34 and 35 is chosen to be sufficient to allow the return of the receiver pulse R during this interval. Commonly, the time interval may be chosen to be 100 microseconds in a system in which the interval between transmitter pulses is 1000 microseconds.

The step voltage output of multivibrator 33 through buffer amplifier 36 to the sampling amplifiers 25a and 25. When the step voltage goes up, as at 34, the voltage E from movable contact 23 is sampled. This voltage is applied through the sampling amplifier 25 to the capacitor 37. As a result, the voltage across capacitor 37 rises linearly toward the voltage E the voltage across capacitor 37 is shown in FIG. 21''. Similarly, when the step function of FIG. 2d goes positive, the voltage E from movable contact 22 is applied through sampling amplifier 25a to the capacitor 33. As a result, the voltage across capacitor 38, as shown in FIG. 22, increases linearly toward E in a negative direction.

The voltage across capacitor 37, shown in FIG. 2 is applied through buffer amplifier 39 to the vertical deflection plates of the plan position indicator 26. Similarly, the voltage across capacitor 38, shown in FIG. 22, is applied through buffer amplifier 40 to the horizontal deflection plates. As a result of the application of the voltages of FIGS. 2e and 2] to the horizontal and vertical deflection plates of the plan position indicator, the electron beam of the plan position indicator will trace out a sweep along the path 27.

When the receiver 12 receives the echo from the trans mitter pulse, the waveform of the received pulse is indicated at R in FIG. 2c. This pulse is applied through amplifier 12a to the CRT cathode of the plan position indicator 26 to turn on the beam so that on its face will appear a spot as at 28, FIG. 2g.

As the transmitter 11 and receiver 12 are rotated in the borehole, successive transmitter pulses initiate successive sweeps on the plan position indicator 26 at increasing values of 0. On each sweep the beam is turned on when the receiver 12 receives the reflection of the transmitter pulse. This causes the beam to register on the display medium of the plan position indicator 26. The result is that a visible trace 29 is registered on the face of the plan position indicator 26, FIG. 2g, thereby indicating the diameter of the borehole at that particular depth. Furthermore, the trace has discontinuities caused by the absence of reflections. These discontinuities are indicative of borehole anomalies.

There can now be appreciated a further aspect of this invention. The provision of the sampling amplifiers 25a and 25 and capacitors 37 and 38 for developing the vertical and horizontal deflection voltages represents a great improvement over prior art techniques for developing the deflection voltages. In the prior art where provision has been made for a potentiometer, such as potentiometer 24, to rotate in synchronism with the rotation of the downhole tool, the vertical and horizontal deflection voltages were developed directly from the potentiometer by applying a sawtooth waveform directly to the potentiometer. By applying a sawtooth voltage, for example, across the points 24a and 24b of the potentiometer, there is develis applied oped directly across the movable contacts 22 and 23 vertical and horizontal sweep voltages which vary in accordance with the angular position of the downhole tool. However, where a very small diameter bore is being calipered, it is necessary that the sawtooth waveform have a period on the order of 100 microseconds. It is not feasible to apply a sawtooth waveform having this small a period, or this high a frequency, to a potentiometer without undue distortion of the waveform. Since potentiometers inherently have a certain capacitance, this capacitance distorts high frequency sawtooth waveforms.

The circuitry making up the buffer amplifier 36, sampling amplifiers 25a and 25, capacitors 37 and 38, and buffer amplifiers 39 and 40 will now be described in detail. Referring to FIG. 3, a negative sync pulse from delay generator 32, FIG. 2, is applied through input diode 41 and capacitors 42 and 43 to the base of transistor 44 which together with transistor 45 forms the multivibrator 33 shown in FIG. 2. The transistor 44 is normally conducting but when the negative sync pulse is applied to the base thereof, the transistor is rendered non-conducting. As a result, the collector of transistor 44 goes positive and this positive-going voltage is coupled through capacitor 46 to render the normally non-conductive transistor 45 conductive. When the transistor 45 is rendered conductive, the collector of transistor 45 goes negative. The time during which the transistor 45 remains in the conductive state is determined by the time constant of the coupling circuit formed by the capacitor 47 and resistor 48 connected between the collector of transistor 45 and the base of transistor 44. When the transistor 45 returns to its non-conductive condition, the voltage at the collector thereof goes positive.

The negative-going step voltage formed at the collector of transistor 45 as a result of the monostable multivibrator action is coupled through capacitor 49 to the base of transistor 59 which is connected as an emitter follower. The emitter follower, The emitter of transistor 50 is coupled through capacitor 51 to the base of transistor 52. Transistors 50 and 52 in FIG. 3 form the buffer amplifier 36 in FIG. 2.

The waveform produced at the collector of transistor 52 is a positive-going step voltage which is applied to the sampling amplifiers. This positive step voltage is applied through capacitor 53 and resistor 54 to the base of the normally conducting transistor 55. Transistor 55 normally provides a conductive path to ground across the capacitor 37. However, when the step voltage at the base of transistor 55 goes positive, the transistor 55 is cut off. At this time, the voltage E from movable contact 23, is applied through resistors 56 and 57 to capacitor 37. The voltage across capacitor 37 begins to rise toward the voltage E The linear voltage across capacitor 37 is applied to the input of buffer amplifier 39 by way of the base of transistor 58 which together with transistor 59 form what is commonly referred to as a Darlington emitter follower circuit. The vertical deflection voltage is developed at the emitter of the transistor 59.

In a similar manner the positive-going step voltage is applied to similar circuitry to develop the horizontal deflection voltage.

There will now be described several modifications which may be made both in the manner of generating the signals indicative of the angular position of the downhole instrument and in the display presentation of the returns from the downhole instrument. First, the means for generating the signals downhole to indicate the angular position of the tool may be modified as shown in FIG. 4 When potentiometers 15 and 18 are of the type shown in FIG. 2, erratic operation of the servo system may result when the movable contact crosses the dead spot in the potentiometer, Quite often the inertia is sufficient to carry the movable contact over the dead spot. However, if the movable Contact 17 of potentiometer 18 stops between the point 19 and the corresponding point at the other end of the potentiometer, the balancing amplifier 20 will always be in balance. Further rotation of the movable contact of the downhole potentiometer 15 will not cause the uphole amplifier 20 to become unbalanced and, therefore, the movable contact 17 will not be moved off the dead spot to follow the movement of the movable contact of the downhole potentiometer 15.

This disadvantage may be overcome by replacing the downhole potentiometer 15 with a continuously wound potentiometer 101 as is shown in FIG. 4. Similarly, the uphole potentiometer 18, FIG. 2, may be replaced with the continuously wound potentiometer 102, shown in FIG. 4. There are no dead spots on the continuously wound potentiometers 101 and 102 and the system will continue to operate regardless of where the movable contact stops. The voltage on the movable contact of potentiometer 101 is applied over line 103 to the uphole potentiometer 102. In FIG. 4, the slowly varying DC voltage from the movable contact of thedownhole potentiometer is applied as one input to the balancing amplifier '104. The other input to balancing amplifier 104 is the voltage on the movable contact of the uphole potentiometer 102. The output of amplifier 104 drives the motor 105 in a direction which moves the movable contact of potentiometer 102 in a direction to balance the voltage inputs to amplifier 104.

The voltage on the movable contact of the downhole potentiometer 101 changes in the manner shown in FIG. 4a as the downhole transmitter and, hence, the movable contact of potentiometer 101 is rotated angularly in the borehole. As the movable contact of potentiometer 101 rotates in a clockwise direction, the voltage of FIG. 4a is at ground when the movable contact is at the point 101a, increases to a positive maximum when the movable contact is at the point 101b, is at ground again when the movable contact is at the point 101a, and decreases to a negative maximum when the movable contact is at the point 101d.

The arrangement of potentiometers shown in FIG. 4 has the disadvantage that when the movable contact passes the point 101b, for example, there is a tendency for the movable contact of the uphole potentiometer 102 to change directions. That is, after the voltage on the movable contact of the potentiometer 101 reaches the point b and starts to decrease so that the voltage on the movable contact reaches the voltage 2, the uphole servo system may tend to balance the system just as it would if the movable contact were at the position producing the voltage at point Since the voltages at points e and f are the same, the uphole servo system has no way of distinguishing between them and may reverse the direction of rotation of the movable contact of potentiometer This disadvantage can be eliminated by using the po tentiometers of the type shown in FIG. 5. In FIG. 5, the downhole potentiometer is of the type having two movable contacts. Such a potentiometer is indicated at 106 in FIG. 5. Similarly, an uphole potentiometer 107, having two movable contacts, is provided. The potentiometers 106 and 107 are of the type in which the two movable contacts rotate concurrently. The advantage of a system using such potentiometers is that when one movable contact is on a dead spot, the other contact is not on a dead spot and continues to drive the servo system.

The movable contact 106a is electrically connected to amplifier 108 which drives motor 100 to balance the movable contact 107a. Similarly, the contact 10617 is electrically connected to amplifier 110 which drives motor 111 to position movable contact 107b.

In operation, the motor 109 tends to drive movable contact 107a to the same position as that assumed by movable contact 106a. Similarly, the motor 111 tends to drive movable contact 107b to the same position as that as sumed by movable contact 101%. The outputs of both motors are mechanically linked together and drive the 10 movable contacts 107a and 107b concurrently. At the same time, they drive the movable contacts of the sinecosine potentiometer 112 which corresponds with the sine-cosine potentiometer 24 in FIG. 2.

The system shown in FIG. 5 does not have the disadvantage of the system shown in FIG. 2 in that when one of the movablec ontacts is on a dead spot, the other movablec ontact will continue to drive the system. The system of FIG. 5 does not have the disadvantage of the system shown in FIG. 4 in that there is no ambiguous position on the potentiometer at which the system may erroneously change direction of rotation.

There will now be described a modification of the presentation of the display of information to provide a more useful, easily interpreted display. In describing the system of FIG. 2, it was assumed that a picture was taken of the face of the plan position indicator 26 when there had been completed one rotation of the assembly including transmitter 11 and receiver 12 around the borehole at a particulard epth. A picture of the PPI scope when one such revolution had been completed would appear as in FIG. 2g indicating the shape of the borehole at a particular depth, together with the position of an anomaly, such as 30, which occurred at that depth. A more useful presentation of information can be obtained if successive traces are recorded on the picture at differing successive depths and, further, if each successive trace has a different diameter. Thus, there can be obtained a picture of the borehole having the three-dimensional effect shown in FIG. 6. In FIG. 6, the trace 29 of FIG. 2g may be the outermost trace. When the logging tool is lowered an incremental amount, a second trace 113 is produced. Similarly, the logging tool is lowered another increment and the trace 114 is recorded at that depth. Similarly, the tool is lowered further increments and the traces 115, 116, 117 and 118 are produced. (As will be apparent from the following description, the logging can be performed equally well while the tool is being continuously raisd in the borehole, and most frequently logging will be performed in this manner. Under this condition, traces 29 and 113-118 will become a continuous spiral.) The net effect of these multiple traces is to produce a three-dimensional effect on a photograph from which the shape of the borehole can be determined and, further, from which the extent and posi tion of a fault in the borehole can be determined. Photographs similar to FIG. 6, and having a great many more traces than that shown in FIG. 6, have a three-dimensional effect which is approximately the same as that obtained from looking directly down the borehole. In FIG. 6, it is quite easy to trace the path of a fault in the borehole as indicated by the discontinuities 119 in the traces.

Photographs similar to that shown in FIG. 6 can readily be obtained with a slight modification of the system of FIG. 2. There is shown in FIG. 2 a delay generator 32, the function of which has not been described previously. The function of the delay generator 32 is to delay the sync pulse 'by different incremental amounts for different depths of the borehole, so that the diameter of the traces 29, 113, 114, 118 will be different for different depths.

It will be appreciated that the diameter of the trace is determined by the time interval between the initiation of the sweep, which occurs when delay generator 32 produces an output, and the arrival of the reflected pulse at the receiver 12. The sweep which produced trace 29 was initiated coincidentally with the transmitter pulse T shown in FIG. 6a. Therefore, the diameter of the trace 29 is determined by the time interval between the transmitter pulse T and the arrival of the receiver pulse R at all points around the periphery of the borehole. Now, if the initiation of the sweep is delayed from the transmitter pulse and is initiated by the pulse S shown in FIG. 6e, the diameter of the resulting trace will be decreased. This results from the fact that the time interval between the pulse S and the receiver pulse R is always less than the interval between the transmitter pulse T and the receiver pulse R.

Therefore, if the sync pulse S can be delayed by successively increasing increments, the diameters of the resulting traces 113-118 will be successively smaller. The delay of the sync pulse S for successively lgreater intervals is the function of delay generator 32.

The circuitry making up the delay generator 32 is shown in more detail in FIG. 7. The output of the sync pulse generator 11a in FIG. 2, corresponding with the transmitter pulse T in FIG. 6a, fires the sync multivibrator 120. The sync tmultivi brator 120 produces the Waveform shown in FIG. 6b. This multivi brator 120 triggers a delay multivibrator 121 thereby producing the waveform shown in FIG. 6c. The short gate of the multivibrator 121 is applied through buffer amplifier 122 to the delay sweep generator 123 which generates a linear ramp voltage which begins with the leading edge of the output of delay multivibrator 121 and terminates with the trailing edge of that gate. The output of the delay sweep generator 123 is applied to the anode of the diode 124. To the cathode of diode 124 there is applied a voltage proportional to the depth of the logging tool. A potentiometer 125 develops a voltage proportional to the depth of the logging tool. As the depth of the logging tool is changed, the movable contact of potentiometer 125 is moved. Thus, there is developed across capacitor 126 a voltage proportional to the depth. The voltage on capacitor 126 determines the voltage at which the diode 124 will become conductive. For example, assume that the voltage across capacitor 126 is volts. The diode 124 will not conduct until the output of delay sweep generator 123 exceeds 5 volts, the waveform being shown in FIG. 6d. When this occurs, the diode 124 conducts a positive voltage which triggers the pulse amplifier 127 to produce the pulse S of FIG. 6e. The time of occurrence of this pulse varies in accordance with the voltage on capacitor 126. As a further example, assume that the voltage on capactor 126 is +7 volts. Then, the diode 124 will not conduct until the output of delay sweep genenator 123 exceeds +7 volts, the waveform being shown in FIG. 6a. This occurs at a later time than when the voltage on the capacitor 126 was only +5 volts. Therefore, the pulse S is delayed by increasing increments of time by increasing the voltage on capacitor 126. As the depth of the logging tool is increased, the voltage across capacitor 126 is increased thereby delaying the initiation of the sweep of the plan position indicator. This produces traces of ever decreasing concentric diameters such as are shown in FIG. 6. In this way, there has 'been produced a three-dimensional representation of the inside diameter of the borehole.

A different presentation of information, which will disclose the path of a fault quite well but which will not indicate the diameter of the borehole, is shown in FIG. 8. In FIG. 8, each of the traces 131-439 represents a different depth of the borehole. Angular position around the periphery of the borehole is indicated by increasing distance to the right. The relation of FIG. 8- to the borehole can be better understood if the paper were to be formed into a cylinder so that point 131b touches point 131a and the extreme right-hand points of the other traces touch the extreme left-hand points of the traces. The cylinder then represents the borehole. The presentation of FIG. 8 is similar to a developed view of the inside of the borehole at different depths.

The presentation of FIG. 8 can be obtained with the equipment shown in FIG. 2 with slight modifications thereof. In FIG. 2, it was assumed that the logging tool remained at one depth in the borehole while a complete rotation of the tool was made. The-n, the depth of the logging tool was changed for another complete rotation of the logging tool in the borehole. Normally, the depth of the logging tool will not be changed in increments but will be continuously changed while the tool is being rotated. This slight modification of the operation of the equipment can be understood with reference to the equipment which produces the traces of FIG. 8.

The modifications of FIG. 2 which will produce the display of FIG. 8 wi'.l now be described. The voltage on the lead 16a from the movable contact of the downhole potentiometer 15 is applied directly to the horizontal deflection plates of an oscilloscope. The voltage on the lead 16a will be a sawtooth waveform as is shown in FIG. 8a. The voltage in FIG. 8a rises to a positive maximum then falls rapidly to zero; rises to a positive maximum again then falls rapidly to zero. The linear rise to a positive maximum will sweep the beam of the oscilloscope across the face of the tube to produce a trace such as the trace 131. The receiver 12 turns on the trace each time it receives an echo. As long as the receiver 12 is receiving echoes, the trace 131 across the face of the oscilloscope will be solid. However, when the transmitter rotates to an angular position at which there is a fault, there will be no return and there will be resulting discontinuities in the traces as at 1310.

As the cathode ray beam is swept across the oscilloscope, the vertical deflection voltage is continuously changed in correlation with the depth of the logging tool in the borehole. Since the logging tool will commonly be continuously moved vertically through the borehole, the vertical deflection voltage is constantly changing and the traces of FIG. 8 are inclined slightly indicating this continuous change of depth of the logging tool.

Instead of continuously changing the vertical deflection voltage on the oscilloscope, another manner of accomplishing the relative movement between the trace and the film is to continuously change the position of the film which is used to record the oscilloscope display. As is better shown in FIG. 12, the oscilloscope film is driven past the face of the oscilloscope in correlation with change of depth of the logging tool thereby producing a display on the film which is similar to that shown in FIG. 8.

As a further modification of a type of presentation, consider a presentation which will be similar to FIG. 8 in that the inside of the borehole is represented as a folded-out flat sheet but which, in addition to indicating faults, also indicates the diameter of the borehole at all angular positions.

The system shown in FIG. 9 provides such a presentation. The voltage from the movable contact of the downhole potentiometer 15, FIG. 2, is again applied to the horizontal deflection plates of an oscilloscope which is shown at 140.

In FIG. 9a, there is shown the transmitter pulses T and the receiver pulses R. It will be appreciated that the voltage from the downhole potentiometer, which voltage is shown in FIG. 9!], increases linearly and very slowly with respect to the repetition rate of the transmitter pulse. For example, complete rotation of the transmitter tool may take place in one second. This is quite a long time compared to the lOOO-microsecond spacing of the transmitter pulses.

The transimtter pulse from sync generator 11a triggers the syn multivibrator a which triggers the delay multivibrator 141, thereby producing the waveform shown in FIG. 9c. This delay multivibrator 141 returns to its stable state at the time 142 which is selected to occur just prior to the expected arrival of the receiver pulse R. When the multivibrator 141 returns to its stable condition, it triggers multivibrator 143, the output of which is shown in FIG. 9d. This multivibrator 143 remains in its unstable condition for a time which should encompass the expected arrival of the receiver pulse R. The output of multivibrator 143, which output is shown in FIG. 9d, is applied to sweep generator 144 which produces the waveform shown in FIG. 92. The total voltage excursion of the sweep voltage of FIG. 9e is a small increment of the total sweep voltage of the downhole potentiometer, as shown in FIG. 9b. For example, if the voltage from the downhole potentiometer increases from O to 10 volts, then the output of the sweep generator 144 might increase from O to 1 

